FIG. 1 shows the conventional fastening structure provided on a plug connector and a corresponding receptacle. As shown, a main body 5 of the conventional plug connector includes a forward projected plug head 51 for plugging in a plug hole 41 on the receptacle 4, so that a closed circuit is formed to allow signal transmission. A fastening element 52 is movably connected to a top of the main body 5. A rear end of the fastening element 52 has a push head 522. A spring 523 is mounted between the push head 522 and the main body 5. At least one hook 521 is provided at a front end of the fastening element 52 for engaging with a locating hole 42 correspondingly formed on the receptacle 4 near the plug hole 41, so that the main body 5 of the plug connector could be firmly held to the receptacle 4. When it is desired to separate the plug connector main body 5 from the receptacle 4, simply downward push the push head 522 of the fastening element 52 to lift and disengage the front hook 521 from the locating hole 42 of the receptacle 4. At this point, the plug head 51 of the connector main body 5 may be easily unplugged from the plug hole 41 of the receptacle 4.
The above-described fastening structure for plug connector has the following disadvantages in use:    1. A height by which the front hook 521 of the fastening element 52 is lifted is in direct ratio to the distance by which the push head 522 is downward pushed. However, as a result of the miniaturization of electronic products, it is not easy to increase the distance between the push head 522 and the main body 5.    2. To meet the requirement of miniaturized electronic products, the fastening element 52 must have a minimized thickness, which inevitably reduces the structural strength of the fastening element 52 to result in easy deformation of the fastening element 52. The lift height of the hook 521 fails to be in direct ratio to the depressed distance of the push head 522 when the fastening element 52 is deformed, preventing the hook 521 from efficiently disengaging from the locating hole 42.    3. The push head 522 is pushed in a direction normal to the direction in which the connector main body 5 is unplugged from the receptacle 4. It is inconvenient for a user to grip the main body 5 and unplug the plug head 51 from the plug hole 41 of the receptacle 4 while press and hold the push head 522.
It is therefore tried by the inventor to develop a plug connector with improved fastening structure that enables firm connection and easy disengagement of the plug connector to and from a receptacle.